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We investigated the interaction among basin-bounding faults, basin fi ll, and geomorphic features of the southern Gar Basin, one of only two known releasing double-bend basins along the Karakoram fault, to better understand their structural evolution and role in basin development. The southern Gar Basin is bounded by an ~44-km-long, N20°Wstriking central fault segment fl anked by two N40°W-striking segments that parallel the regional strike of the Karakoram fault system. The central fault segment is composed of a system of strike-slip and normal faults that young basinward and incorporate basin fi ll in their uplifted footwalls. The oldest faults along the extensional portion of the bend are dominantly strike-slip, and they strike ~15°W from the main strike of the Karakoram fault. Basin fi ll is broadly folded about a NNWtrending axis and can be explained by E-SEdirected slip along a listric normal fault. Cross sections across the basin and associated faults suggest the geometry is best described as an extensional fl ower structure. Forward structural modeling of the intrabasinal faults shows that the system has accommodated ~8 km of east-west extension. We interpret the bend to have formed from linking of R and P shears into a through-going principal displacement zone. At shallow levels in the crust (low confi ning pressures), R shears are exploited; at deeper levels, these faults merge with the principal displacement zone, forming the extensional fl ower structure geometry. We estimate that the shear zone is 50-35 km wide based on the aerial distribution of P and R shears. Restoration of R shears on the west and east sides of the Gar Valley indicates ~55 km of right-lateral separation along the Karakoram fault, which is a minimum slip estimate for the Karakoram fault system.

The timing of geologic events along the India-Asia suture in southern Tibet remains poorly understood because minimal denudation prevents widespread exposure of structurally deep rocks. In this study, we present geologic maps of two structurally deep domes, cored by mylonitic orthogneisses, across the India-Asia suture zone in southwestern Tibet. New U-Pb zircon ages and rock textures indicate that core ortho gneisses are originally Gangdese arc rocks that experienced Late Eocene prograde metamorphism, probably during crustal thickening. Crosscutting leucogranite sills underwent northwest-southeast extension related to slip along a brittleductile shear zone here designated the Ayi Shan detachment. The timing of shear along detachment is bracketed by zircon U-Pb ages of 26-32 Ma for these pre-to syn(?)extensional leucogranites, and by a 40 Ar/ 39 Ar muscovite age of 18.10 ± 0.05 Ma for a rhyolitic dike. This rhyolite dike crosscuts a widespread siliciclastic unit that was deposited across the detachment, which we correlate to the Kailas Formation. The Great Counter thrust defi nes the surface trace of the India-Asia suture zone; it cuts the Kailas Formation, and is in turn cut by the Karakoram fault. A new 40 Ar/ 39 Ar muscovite age of 10.17 ± 0.04 Ma for the Karakoram fault footwall is consistent with published thermochronologic data that indicate Late Miocene transtension in southwestern Tibet.

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